I. Field of the Invention
This invention relates generally to the design of cardiac stimulating devices and, more particularly, to an implantable monitor/stimulator that monitors and assesses level of cardiac function then permits a physician to arbitrate the therapy mode, if therapy is indicated. It accomplishes this by assessing impedance, electrocardiogram, and/or pressure measurements, then calculating various cardiac parameters. The results of these calculations determine the mode of therapy to be chosen, then therapy may be administered by the device itself or a control signal may be telemetered to various peripheral devices aimed at enhancing the heart's function. Alternatively, the device may be programmed to monitor and either store or telemeter information without delivering therapy.
II. Discussion of the Prior Art
Patients suffering from chronic congestive heart failure manifest an elevation of left ventricular end-diastolic pressure, according to the well-known heterometric autoregulation principles espoused by Frank and Starling. This may occur while left ventricular end-diastolic volume remains normal due to a decrease in left ventricular compliance concomitant with increased ventricular wall stiffness. Prior attempts to increase wall contractility and thus improve cardiac performance have focused on drug therapy and cardiomyostimulation.
Many inotropic drugs have recently become available, targeted at various receptors in the ventricular walls and designed for the purpose of directly stimulating cardiac tissue in order to increase contractility. However, there exist many possible undesirable side effects, in addition to the fact that these drugs do not always work for their intended purpose. This is especially characteristic of the patient suffering from end-stage heart failure. Because of these problems with drug efficacy, the techniques of adaptive rate cardiac pacing and cardiomyostimulation have been developed.
The performance of adaptive rate cardiac pacemakers has advanced greatly in recent years. Such pacers sense the presence or absence of intrinsic cardiac electrical activity, or other physiologic parameter, and then respond only by increasing or supporting heart rate by direct stimulation of cardiac tissue. The heart rate response may also be based on the sensing of some type of physiologic need but, to date, no implantable device has been proposed to directly evaluate cardiac function and then deliver appropriate therapy designed to improve function. Although adaptive rate pacing may increase cardiac output by increasing heart rate, it has not been indicated as a therapy in heart failure because neither contraction nor relaxation are improved, and conversely, increased myocardial oxygen demands may ensue. The only application of pacing technology to address heart failure has been in the area of cardiomyoplasty, where electrical stimulation is directed towards some type of skeletal muscle system to augment cardiac function.
Cardiomyostimulation is a technique intended to increase cardiac output, in order to assist a compromised heart. As disclosed in U.S. Pat. No. 4,735,205, issued to Chachques, skeletal muscle can be trained to withstand the rigor of long-term sequential contraction without undue fatigue. When such trained muscle is surgically wrapped around the ventricles then sequentially electrically stimulated using demand-type cardiac pacer circuitry, mechanical assistance is provided for compromised contraction. This occurs because stimulated contraction of this skeletal muscle causes constriction upon the ventricles and forces blood into the arterial system. Although this process has proven to be useful, it does not directly affect contractile forces within the heart itself and is known to impair diastolic function by increasing ventricular chamber "stiffness".
Despite the improvements in treatment described above, there remains a large group of patients for whom these approaches either do not work or are contraindicated for other medical reasons. The present invention is intended to enhance contractility or relaxation in this group of patients by providing an apparatus that monitors cardiac function and then directly stimulates ventricular tissue in a way that optimizes the functional parameter or parameters under control.
Impedance-based measurements of cardiac parameters such as stroke volume are known in the art. U.S. Pat. No. 4,674,518, issued to Salo, discloses an impedance catheter having plural pairs of spaced surface electrodes driven by a corresponding plurality of electrical signals comprising high frequency carrier signals. The carrier signals are modulated by the tidal flow of blood in and out of the ventricle. Raw signals are demodulated, converted to digital, then processed to obtain an extrapolated impedance value. When this value is divided into the product of blood resistivity times the square of the distance between the pairs of spaced electrodes, the result is a measure of blood volume held within the ventricle. These calculations may be made using spaced sensors placed within a catheter, as in the Salo '518 patent, or they may be derived from signals originating in electrodes disposed in the heart, as described in U.S. Pat. No. 4,686,987, issued to Salo and Pederson. The device of the '987 patent senses changes in impedance to determine either ventricular volume or stroke volume (volume of blood expelled from the ventricle during a single beat) to produce a rate control signal that can be injected into the timing circuit of another device, such as a cardiac pacer or drug infusion pump. In this manner, the rate of operation of the slaved device may be controlled. An example of application of this impedance sensing circuitry to a demand-type cardiac pacer is disclosed in U.S. Pat. No. 4,773,401, issued to Citak, et al. Other devices may combine impedance sensing with internal pressure measurement as disclosed in U.S. patent application Ser. No. 07/490,392 of Salo, and with telemetry as disclosed in U.S. Pat. No. 4,562,841, issued to Brockway, et al.
The present invention combines these approaches, rendering a device that detects and monitors levels of cardiac function and delivers therapy on the basis of this monitored information. The primary mode of delivery is direct electrical stimulation, resulting in improved contractility, relaxation or improved cardiac output.